The present invention relates to a developing device for supplying a developer to the surface of a photosensitive body on which an electrostatic latent image is to be formed and developing the electrostatic latent image, more specifically to a developing device disposed at a distance from the photosensitive body.
In a conventional device of a well-known type for developing an electrostatic latent image, developing means including a fur brush, cascade, magnetic brush, etc., are brought directly into contact with the surface of a photosensitive body. In such a developing device, a developer is supplied at random to the surface of the photosensitive body on which the electrostatic latent image is to be formed. Therefore, the developer cannot help sticking to that portion of the surface of the photosensitive body on which the electrostatic latent image is not formed (no-picture region) and which must be kept away from the developer, as well as to that portion on which the electrostatic latent image is formed (picture region). The developer sticking to the no-picture region, when fixed, would render a resultant copy image or picture foggy, thus exerting a bad influence upon the copy image quality.
Accordingly, there has recently been proposed a developing device in which a developing means, e.g., a developing roller, is separated from the surface of the photosensitive body, and the developer is supplied selectively to the picture region of the photosensitive body in accordance with the surface potential of the electrostatic latent image. According to the developing device of this type, the developer is not fed to the no-picture region, so that no fogging will be caused in the no-picture region to ensure satisfactory copy image quality.
In a copying apparatus with a non-contact type developing device, however, the photosensitive body and developing roller must be so designed as to move in the same direction at equal speeds in order to make the copy density microscopically. To this end, first, the moving speed of the photosensitive body is determined in proportion to the moving speed of the original (or the moving speed of an exposure lamp) on the basis of the specifications of the copying apparatus. On condition of the equality of speed, the moving speed of the developing roller is then determined. In the determination of the moving speed of the developing roller, therefore, it is impossible to take the developing characteristics into consideration, though it is essential to make an effort to maintain the developing characteristics in accordance with the previously determined moving speed of the developing roller. Thus, the developing device of the non-contact type is subject to difficulties in design and adjustment work, and is not suited for industrial use.